A wide variety of mental and physical processes are controlled or influenced by neural activity in particular regions of the brain. In some areas of the brain, such as in the sensory or motor cortices, the organization of the brain resembles a map of the human body; this is referred to as the “somatotopic organization of the brain.” There are several other areas of the brain that appear to have distinct functions that are located in specific regions of the brain in most individuals. In the majority of people, for example, areas of the occipital lobes relate to vision, regions of the left inferior frontal lobes relate to language, and regions of the cerebral cortex appear to be consistently involved with conscious awareness, memory, and intellect. This type of location-specific functional organization of the brain, in which discrete locations of the brain are statistically likely to control particular mental or physical functions in normal individuals, is herein referred to as the “functional organization of the brain.”
Many problems or abnormalities can be caused by damage from disease and/or disorders of the brain. A stroke, for example, is one common condition that damages the brain. Strokes are generally caused by emboli (e.g., obstruction of a vessel), hemorrhages (e.g., rupture of a vessel), or thrombi (clotting) in the vascular system of a specific region of the cortex, which in turn generally causes a loss or impairment of neural function (e.g., neural functions related to face muscles, limbs, speech, etc.). Other problems or abnormalities can be caused by traumatic brain injury, memory diseases/disorders (e.g., Alzheimer's, dementia, etc.), movement disorders, tinnitus, neuropsychiatric and/or neurocognitive disorders, addictions, and/or other conditions.
Several existing techniques for treating various conditions involve electrically stimulating certain regions of the brain. In stroke cases, for example, Northstar Neuroscience, Inc., has pioneered electrically stimulating selected regions of the cortex to treat stroke-related conditions. Northstar Neuroscience has discovered that stimulating selected areas of the cortex below the activation threshold of a population of neurons at the stimulation site is beneficial in treating such conditions. For example, the activation threshold can be the minimum electrical current that triggers a motor response or a sensation, and the therapeutic electrical current can be applied at approximately 50% or another subthreshold level of the movement or sensation current. Therefore, it is useful to first determine the electrical current that induces a movement and/or sensory response at the stimulation site to effectively conduct subthreshold stimulation therapies.
One challenge of conducting subthreshold stimulation therapies is determining the activation threshold for the stimulation site. As mentioned above, the therapy level can be based on the minimum level of current that induces movement in the patient's affected body part and/or a sensation perceived by the patient. The minimum activation current level is generally determined by applying the stimulation at increasing current or voltage levels until the stimulation itself evokes a movement, a sensation, and/or another type of measurable response in the patient. The implantable pulse generators, however, have only a limited output capacity that in certain situations may not be sufficient to evoke such a response. When this occurs, it is difficult to estimate the minimum activation current, and thus the stimulation may not be applied in a desired subthreshold stimulation range.
Another aspect of applying electrical stimulation to the cortex is to provide a sufficient current over a desired area of the cortex to achieve an intended effect and/or drive the electrical signal to a desired depth within the cortex. For example, it may be advantageous to stimulate an area that covers portions of the sensory cortex, motor cortex, and/or pre-motor cortex. This generally requires an electrode array with a plurality of contacts (e.g., a 2×3 electrode array). This can also require a higher current level to achieve an intended effect. Such a current level may exceed the maximum output of the implantable pulse generator. As a result, the implantable pulse generator may not be adequate for performing certain therapies.